1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device with a column spacer.
2. Discussion of the Related Art
Much effort has been devoted to research and develop various display devices to address the demand of information-driven society. In particular, flat panel display devices are in high demand. They include liquid crystal display (LCD) device, plasma display panel (PDP), electroluminescent display (ELD) and vacuum fluorescent display (VFD). These flat panel display devices have already been applied in some form to various display equipment.
Among the various flat display devices, the LCD device has been most widely used because it is compact, thin, and consumes low power. The LCD device is increasingly being used as a substitute for Cathode Ray Tube (CRT). In addition to being used on mobile devices such as notebook computers, LCD devices have been developed as computer monitors and television displays.
Despite various technical advances in LCD technology with applications in different fields, research in picture quality enhancement of the LCD device has been lagging other features and advantages of the LCD device. Whether LCD devices become ubiquitous as a general purpose display, will depend on their capabilities in achieving high quality picture, such as high resolution and high luminance with a large screen while remaining light, thin, and low power consuming.
FIG. 1 is an exploded perspective view of a related art LCD device. Referring to FIG. 1, the related art LCD device includes first and second substrates 1 and 2, and a liquid crystal layer 3 formed by injection between the first and second substrates 1 and 2. Specifically, the first substrate 1 includes a plurality of gate lines 4 arranged along a first direction at fixed intervals, a plurality of data lines 5 arranged along a second direction perpendicular to the first direction at fixed intervals, a plurality of pixel electrodes 6 arranged in a matrix-type configuration within pixel regions P defined by crossing of the gate and data lines 4 and 5, and a plurality of thin film transistors T at respective crossing points of the gate and data lines 4 and 5. The thin film transistors T apply a data signal from the data line to each pixel electrode 6 according to a gate signal of the gate line. The second substrate 2 includes a black matrix layer 7 that shields light from the predetermined portions of the first substrate 1 except for the pixel regions, an R, G and B color filter layer 8 for representing various colors in correspondence with the pixel regions, and a common electrode 9 on the color filter layer 8 to render images.
In the related art LCD device, since the liquid crystal layer 3 is formed between the first and second substrates 1 and 2, liquid crystal molecules of the liquid crystal layer 3 are driven by an electric field generated between the pixel electrode 6 and the common electrode 9. For example, an alignment direction of the liquid crystal molecules of the liquid crystal layer 3 is controlled by the induced electric field between the pixel electrode 6 and the common electrode 9. Accordingly, the amount of light transmitted through the liquid crystal layer 3 may be controlled by the alignment direction of the liquid crystal molecules, thereby displaying images. The related art LCD device described above is commonly referred to as a Twisted Nematic (TN) mode LCD device, which has a narrow viewing angle.
In order to overcome the problem associated with the TN mode LCD device, an In-Plane Switching (IPS) mode LCD device has been developed. In the IPS mode LCD device, a pixel electrode and a common electrode are formed in parallel at a predetermined interval within a pixel region. Accordingly, an electric field parallel to the substrates is generated between the pixel electrode and the common electrode, thereby aligning liquid crystal molecules of a liquid crystal layer parallel to the substrates.
A method of fabricating a related art IPS mode LCD device will be described as follows. Generally, the method of fabricating the LCD device is categorized as a liquid crystal injection method and a liquid crystal dispensing method, according to how the liquid crystal layer is formed between the two substrates.
FIG. 2 is a flow chart of a method of fabricating a liquid crystal injecting type LCD device according to the related art. The method for fabricating an LCD device is divided into three processes, including an array process, a cell process, and a module process. The array process mainly includes two steps: forming a TFT array having gate and data lines, common electrodes, and thin film transistors on the first substrate; and forming a color filter array having a black matrix layer, a color filter layer, and a common electrode on the second substrate.
During the array process, a plurality of LCD panels are formed together on one large mother glass substrate. The TFT array and the color filter array are formed on each of the LCD panels. Then, the TFT substrate and the color filter substrate are moved to a cell process line. Subsequently, an alignment material is coated on the TFT substrate and the color filter substrate. Then, an alignment process (rubbing process) S10 is performed on the substrates to obtain a uniform alignment direction of liquid crystal molecules. The alignment process S10 is carried out by the following steps: cleaning the substrate before coating an alignment layer thereon, printing the alignment layer, baking the alignment layer, inspecting the alignment layer, and rubbing the alignment layer. Then, the TFT substrate and the color filter substrate are respectively cleaned (S20).
Next, ball spacers for maintaining a cell gap between the two substrates are scattered on one of the two substrates (S30). A seal pattern is formed corresponding to the circumference of respective LCD panel regions to bond the two substrates to each other (S40). The seal pattern includes an inlet through which liquid crystal is injected. In this case, the ball spacers may be formed of plastic balls or minute elastic particles. Then, the TFT substrate and the color filter substrate having the seal pattern therebetween are positioned facing each other, and are bonded to each other. Then, the seal pattern is cured (S50).
Thereafter, the bonded TFT and color filter substrates are cut into respective LCD panel regions (S60), thereby fabricating the unit LCD panels, each having a predetermined size. Then, the liquid crystal is injected to the LCD panel through the inlet, and the inlet is sealed (S70), thereby forming a liquid crystal layer. After performing an inspection process (S80) for observing external appearances and testing for electric failures in the LCD panel, the process of fabricating the LCD device is completed.
During the process for injecting the liquid crystal, the LCD panel and a container having liquid crystal therein are provided in a vacuum chamber. Accordingly, moisture and air bubbles in the liquid crystal and the container are simultaneously removed, and an interior space of the LCD panel is maintained in a vacuum state. Then, the inlet of the LCD panel is dipped into the container having the liquid crystal in the vacuum state, and the vacuum state inside the chamber is changed to the atmospheric pressure. Thus, the liquid crystal is injected into the interior of the LCD panel through the inlet according to a pressure difference between the interior of the LCD panel and the vacuum chamber.
The liquid crystal injection method has the following disadvantages. First, after cutting the large mother glass substrate into the respective LCD panel regions, the inlet is dipped into the container having the liquid crystal while maintaining the vacuum state between the two substrates. Significant amount of time are required for injecting the liquid crystal between the two substrates, thereby lowering yield. When forming large LCD devices, it is difficult to completely inject the liquid crystal into the innermost portion of the LCD panel, thereby causing defect due to incomplete injection of the liquid crystal. Furthermore, several liquid crystal injection devices are required due to the complicated process and the considerable process time, thereby requiring large spaces for housing the several devices. Also, if the ball spacers are used in the LCD device, the ball spacers may lump together, thereby causing a Milky Way defect of generating glitter. Also, the ball spacers are scattered, whereby the ball spacers may be moved within the pixel region, thereby causing light leakage.
In order to overcome these problems of the liquid crystal injection method, the liquid crystal dispensing method has been developed, in which two substrates are bonded to each other after dispensing liquid crystal on any one of the two substrates. FIG. 3 is a flow chart of a method of fabricating a liquid crystal dispensing type LCD device according to the related art. In the liquid crystal dispensing method, before bonding the two substrates, the liquid crystal is dispensed on any one of the two substrates. Accordingly, it is impossible to use ball spacers for maintaining a cell gap between the two substrates since the ball spacers move to a dispensing direction of the liquid crystal. Thus, instead of the ball spacers, patterned spacers or column spacers are fixed to the substrate to maintain the cell gap between the two substrates. During an array process, a black matrix layer, a color filter layer, and a common electrode are formed on the color filter substrate. Then, a photosensitive resin is formed on the common electrode, and is selectively removed to form the column spacers above the black matrix layer. The column spacers may be formed in a photo process or an ink-jet process.
Then, alignment layers are respectively coated on the entire surfaces of the TFT substrate and the color filter substrate including the column spacers, and a rubbing process is performed thereto. After cleaning the TFT substrate and the color filter substrate (S101), the liquid crystal is dispensed on one of the two substrates (S102), and a seal pattern is formed in the circumference of an LCD panel region on the other substrate by a dispensing apparatus (S103). At this time, it is possible to dispense the liquid crystal and form the seal pattern together on one of the two substrates. The other substrate having no liquid crystal dispensed thereon is inversed (S104). Then, the TFT substrate and the color filter substrate are bonded to each other by pressure, and the seal pattern is cured (S105).
Subsequently, the bonded substrates are cut into the respective LCD panels (S106). In addition, an inspection process (S107) for observing external appearances and tests for electric failures in the LCD panel are performed. The process of fabricating the LCD device is complete.
In the liquid crystal dispensing method, the column spacers are formed on the color filter substrate. Liquid crystal is dispensed on the TFT substrate. Then the two substrates are bonded to each other, thereby forming the LCD panel. The column spacers are fixed on the color filter substrate, and are in contact with the TFT substrate. Also, when the column spacers are in contact with the TFT substrate, the contact portion corresponds to one of the gate line and the data line. That is, each of the column spacers is formed on the color filter substrate at a predetermined height.
FIG. 4 is a plane view of a related art LCD device. FIG. 5 is a cross-sectional view along I-I′ of FIG. 4. As shown in FIG. 4 and FIG. 5, an array area of the related art LCD device includes a gate line 4, a data line 5, a thin film transistor TFT, and a pixel electrode 6. At this time, the gate line 4 is formed perpendicular to the data line 5, to define a pixel region, and the thin film transistor TFT is formed at a crossing of the gate line 4 and the data line 5. Also, the pixel electrode 6 is formed in the pixel region. Then, column spacers 20 are formed at fixed intervals, to maintain a cell gap. In this case, as shown in FIG. 5, the column spacer 20 is formed above the gate line 4. That is, the gate line 4 is formed on a first substrate 1, and a gate insulating layer 15 is formed on the entire surface of the first substrate 1 including the gate line 4. Then, a passivation layer 16 is formed on the gate insulating layer 15. Also, a second substrate 2 includes a black matrix layer 7, a color filter layer 8, and a common electrode 14. The black matrix layer 7 is formed on the second substrate 2 to cover the non-pixel portions (gate line, data line, and thin film transistor) except for the pixel regions. The color filter layer 8 is formed on the second substrate 2 including the black matrix layer 7 by forming R, G and B pigments in correspondence with the pixel regions. Then, the common electrode 14 is formed on the entire surface of the second substrate 2 including the color filter layer 8. The column spacer 20 is formed on the common electrode 14 corresponding to the gate line 4. Then, the first substrate 1 and the second substrate 2 are bonded to each other while the column spacer 20 is positioned in correspondence with the gate line 4.
The liquid crystal dispensing type LCD device having the column spacer has the following disadvantages. First, the column spacers are fixed to one substrate, and the flat surface of column spacers is in contact with the TFT substrate, causing a great frictional force due to the large contact surface with the substrates. Accordingly, when the screen of the LCD device having the column spacers is rubbed, it generates touch spots on the screen for a long time.
FIG. 6A and FIG. 6B are a plane view and a cross-sectional view, respectively, illustrating spots generated on the screen by touching the LCD panel. If the LCD panel 10 is continuously touched with a finger or a pen along a predetermined direction, as shown in FIG. 6A, the upper substrate 2 of the LCD panel 10 is shifted at a predetermined interval in the touch direction, as shown in FIG. 6B. If the cylindrical column spacers are in contact with the lower and upper substrates 1 and 2, it may cause a great frictional force between the column spacers and the two opposing substrates. Thus, the liquid crystal molecules between the column spacers are not restored to their original state, thereby generating the spots on the screen. Also, when the LCD panel is touched with the finger or pen along the predetermined direction, the liquid crystal molecules gather around the touched portion causing a protrusion. In this case, the cell gap h1 corresponding to the protruding portion is higher than the cell gap h2 of the remaining portions, thereby generating light leakage. Then, it is impossible to obtain a uniform luminance across the LCD device. In the LCD device of forming the column spacers, the spots may be generated by touch since the column spacers are fixed to one substrate, and the column spacers contact the opposing substrate on a flat surface. Thus, the contact area of the column spacers with the substrates is larger than the contact area of the ball spacers in the related art LCD device described earlier.
Another reason for spots generation is that the contact area between the substrate and the column spacers is in a vacuum state when the substrate is touched. When ball spacers are used, the ball spacers may be moved in all directions if the facing substrate is touched. Thereby, the contact area between the surface of the substrate and the ball spacers is not maintained in the vacuum state when the surface of the LCD panel is touched. In contrast, when the column spacers are used, if the upper surfaces of the column spacers are in contact with the flat surface of the facing substrate, the contact area is in the vacuum state. Accordingly, when the LCD device uses column spacers, the spots are generated when the surface of the LCD panel is touched, due to the large contact area between the column spacers and the opposing substrate, or due to the vacuum state between the flat surface of the substrate and the upper surfaces of the column spacers.
In the LCD device formed with the liquid crystal injection method, the appropriate amount of liquid crystal is injected inside the LCD panel due to the difference in pressure between the inside of the LCD panel, which is in the vacuum state, and the inside of the chamber, which is under the atmospheric pressure, thereby completing the LCD panel. In the liquid crystal dispensing method, the LCD panel is completed by dispensing the predetermined amount of liquid crystal is on one substrate, and then bonding the two substrates are bonded to each other.
Accordingly, in the liquid crystal injection method, the appropriate amount of liquid crystal is injected due to the pressure difference irrespective of the structures formed on the two substrates inside the LCD panel. However, in the liquid crystal dispensing method, it is difficult to predetermine the amount required to each of the LCD panels, due to variations in sizes and intervals for the structures generated by the fabrication margin in the LCD panels. In the liquid crystal dispensing method, the amount of liquid crystal provided to the LCD panel may be excessive. As a result, when excessive liquid crystal is provided, or the liquid crystal is maintained at a high temperature, lower portions of the LCD panel are protruding due to the gravity defect. That is, the liquid crystal molecules may gather to some lower portions of the LCD, because of the excessive amount of liquid crystal provided to the inside of the LCD panel.
Generally, the LCD device is used as a display device for television, notebook and desktop computers. In these applications, the LCD panel of the LCD device usually stands vertically. In this case, the liquid crystal molecules of the LCD panel move and gather to a lower portion thereof due to the effects of gravity. Specifically, when the LCD panel is maintained at the high temperature, the thermal expansion of liquid crystal increases, aggravating the problem. The aforementioned problems of the spots and the gravity defect become more serious in large LCD devices because, then, it is difficult to dispense the liquid crystal on the entire surface of the LCD panel.